It’s not the extraordinarily mild winter that’s to blame, it’s the fluctuation of mouse populations and acorn harvests. (More on that later.) But the mild winter may mean that the danger period, when tiny young “nymph” ticks that carry Lyme are out for our blood, could begin earlier than usual. The bottom line from Dr. Richard S. Ostfeld, a Cary Institute disease ecologist: Watch out. Starting now.

Normally, Lyme disease risk ramps up in May. But “This past winter,” he said “was record-breaking mild, and when you get records, when you have extremes in weather events, to some degree all bets are off. We don’t really know whether the nymphs are going to start their activity earlier this year than in normal years. So it’s remotely possible they could be out as early as April. They’re cold-blooded creatures, so things get speeded up in terms of their metabolism and development when things are warmer. So it could be a bit earlier than usual. I wouldn’t wait to be vigilant. The time is now.” (What does such vigilance look like? Here are prevention tips from the CDC.)

I asked Dr. Ostfeld if he could offer some order of magnitude of the expected surge in Lyme disease. Very, very roughly, he said, we could see perhaps 20 percent more cases than usual.

Acorn production in 2010 set a record for the 20-plus years it has been monitored, he said, “and mouse abundance in the summer of 2011 was oh, perhaps 10 or 20 percent higher than we’ve ever recorded. So I would expect the Lyme disease risk should be at least that much higher than we’ve had in the past.” He emphasized: “This is a prediction based on past trends, and in ecology, as in economics — what do they say about stock portfolios? — past performance does not predict future returns. But that’s my best estimate as to what might happen.”

Now for a bit of the ecology behind the predictions. From the Cary Institute’s press release:

What do acorns have to do with illness? Acorn crops vary from year-to-year, with boom-and-bust cycles influencing the winter survival and breeding success of white-footed mice. These small mammals pack a one-two punch: they are preferred hosts for black-legged ticks and they are very effective at transmitting Borrelia burgdorferi, the bacterium that causes Lyme disease.

“We had a boom in acorns, followed by a boom in mice. And now, on the heels of one of the smallest acorn crops we’ve ever seen, the mouse population is crashing,” Ostfeld explains. Adding, “This spring, there will be a lot of Borrelia burgdorferi-infected black-legged ticks in our forests looking for a blood meal. And instead of finding a white-footed mouse, they are going to find other mammals—like us.”

For more than two decades, Ostfeld, Cary Institute forest ecologist Dr. Charles D. Canham, and their research team have been investigating connections among acorn abundance, white-footed mice, black-legged ticks, and Lyme disease. In 2010, acorn crops were the heaviest recorded at their Millbrook-based research site. And in 2011, mouse populations followed suit, peaking in the summer months. The scarcity of acorns in the fall of 2011 set up a perfect storm for human Lyme disease risk.

Black-legged ticks take three bloodmeals—as larvae, as nymphs, and as adults. Larval ticks that fed on 2011’s booming mouse population will soon be in need of a nymphal meal. These tiny ticks—as small as poppy seeds—are very effective at transmitting Lyme to people. The last time Ostfeld’s research site experienced a heavy acorn crop (2006) followed by a sparse acorn crop (2007), nymphal black-legged ticks reached a 20-year high.

The May-July nymph season will be dangerous, and Ostfeld urges people to be aware when outdoors. Unlike white-footed mice, who can be infected with Lyme with minimal cost, the disease is debilitating to humans. Left undiagnosed, it can cause chronic fatigue, joint pain, and neurological problems. It is the most prevalent vector-borne illness in the U.S., with the majority of cases occurring in the Northeast.

Ostfeld says that mild winter weather does not cause a rise in tick populations, although it can change tick behavior. Adult ticks, which are slightly larger than a sesame seed, are normally dormant in winter but can seek a host whenever temperatures rise several degrees above freezing. The warm winter of 2011-2012 induced earlier than normal activity. While adult ticks can transmit Lyme, they are responsible for a small fraction of tick-borne disease, with spring-summer nymphs posing more of a human health threat.

Past research by Ostfeld and colleagues has highlighted the role that intact forest habitat and animal diversity play in buffering Lyme disease risks. He is currently working with health departments in impacted areas to educate citizens and physicians about the impending surge in Lyme disease.

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Massachusetts is the leading laboratory for health care reform in the nation, and a hub of medical innovation. From the lab to your doctor’s office, from the broad political stage to the numbers on your scale, we’d like CommonHealth to be your go-to source for news, conversation and smart analysis. Your hosts are Carey Goldberg, former Boston bureau chief of The New York Times, and Rachel Zimmerman, former health and medicine reporter for The Wall Street Journal.

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